1. Field of the Invention
This invention relates to a cryogenic refrigerator in which a very low temperature level is generated in a cryostat (low temperature tank) and which maintains a cryostat working apparatus in a refrigerated state by expanding refrigerant gas, such as helium gas, and particularly to a measure of reducing vibration in a cryogenic working apparatus in a cryostat which is maintained at a very low temperature level.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,223,540, a helium refrigerator is well known as a very low temperature refrigerator. This helium refrigerator is provided with a precooling refrigerating circuit, whereby a cryogenic maintaining part in a cryostat is radiantly shielded from the outside by expanding high pressure helium gas by an expander, and a J-T circuit whereby compressed helium gas discharged from another compressor is precooled in said precooling refrigerating circuit and such precooled helium gas is then Joule-Thomson expanded at a J-T valve to generate cold in the cryogenic level maintaining part of the cryostat by expanding action at that time.
Another type of a helium refrigerator which is also known is a helium refrigerator which is provided with such an expander as mentioned above but without a J-T circuit and generates cold in a low temperature level maintaining part of a cryostat by expanding helium gas by the expander.
In such a helium refrigerator, a refrigerating part which is arranged in a low temperature level maintaining part of a cryostat and refrigerates an object for cooling is supported at the lower part of a cylinder of an expander.
However, in such helium refrigerators as mentioned above, a G-M cycle (Gilford-MacMahon cycle), a modified Solvay cycle or the like is generally employed as a refrigerating cycle provided by a precooling refrigerating circuit. In this case, it is inevitable that vibration is generated due to a change of pressure (high pressure/low pressure) in flowing gas at an expander, collision of a displacer with a cylinder, expansion and shrinkage of a cylinder due to change of pressure (high pressure/low pressure), etc. Thus, it was difficult to use such cycles in a system including a photo-detecting sensor to be used in spectrochemical study where micro vibration in the order of .mu.m must be avoided.
From the above, a construction in which an expander and a part to be cooled are supported individually by a cryostat comes under consideration as a solution to the vibration problem. However, such an individual support construction causes difficulty in the setting up of a refrigerator and the only cooling method which can be employed is one in which the working of an expander is stopped when a cryogenic working apparatus is working, and to refrigerate a part to be cooled by cold heat of such liquid helium. Thus, the field of use of refrigerators is limited.